Above figure shows the layout of a three-speed gearbox with a synchromesh system in its second and top gears. The housing, rotating parts with bearings and the shilfüng mechanism constitute the gearbox. The housing is generally made of cast iron and bolted to the rear of the clutch housing. The housing acts as a container for the oil in which the gears operate. The case cover usually carries the shifüng mechanism and it seals the gearbox against water and dirt. The dog clutch, synchronizer assemblies the bearing constitute the rotating parts. The flow of power during the various gear positions is discussed below.
The layout of a 3-speed transmission.
The figure below shows the simplified diagram of a gearbox depicting the flow of power in the neutral position. In this position, the gears inside the gearbox simply rotate without affecting the main shaft of the gearbox. The power transmitted by the clutch simply rotates the main drive gear and countershaft drive gear as well as the second-speed gears on the main shaft and countershaft since they are constantly in a mesh.
The second speed gear on the main shaft is loosely fitted and hence does not drive the main shaft. All the gears on the countershaft including the reverse gear rotate without affecting the main shaft. The bold line depicts the power flow.
The figure below shows the simplified diagram of a gearbox showing a flow of power in first gear. This position is obtained by sliding the first and reverse gears to the left unit, its teeth mesh with that of the countershaft first gear. In this position, a drive is given to the countershaft from the clutch and the main shaft is driven by the first gear, which is splined to it.
The figure below shows the simplified diagram of a gearbox showing a flow of power in second gear. Before obtaining this position, the first gear slides out of mesh bringing the gearbox in the neutral position. After this, the dog clutch is moved to the right side making its internal teeth to mesh with the external teeth of the second speed gear. The dog clutch is splined with the main shaft.
Third or top gear
The figure below shows the simplified diagram of a gearbox showing the flow of power in the third gear. This position is obtained by shifting the dog clutch to the left until its internal teeth mesh with the external teeth of the main drive gear. In this position, the main shaft is locked with the clutch shaft, thus making a direct drive.
The figure below shows the simplified diagram of a gearbox showing the flow of power in the reverse gear. Before obtaining this gear, the vehicle is brought to rest and the gearbox to its neutral position. After this, the first and reverse gear is moved to the left till it meshes with the reverse idler. The second-speed gears revolve but transmit no power. In this position, the power is transmitted through the reverse idler to the main shaft by the first reverse gear which is splined to the main shaft, gears on the main shaft are in constant mesh with the corresponding gears on the countershaft. The reverse idler gear and the reverse sliding gears are not in a mesh. In this position, the gearbox is in neutral since no power is transmitted to the main shaft.
You can better understand this process by watching the video below: